Efficacy of stress-response protease-deficient Salmonella enterica serovar Typhimurium as a candidate of live oral vaccine strain

鼠伤寒沙门氏菌(CS2022)经口服感染途径给药，具有免疫原性。在本研究中，CS2022在单次口服10^7CFU的沙门氏菌后，在小于102CFU的BALB/c小鼠的脾内持续停留了长达6周。在接种后4、6和13周，血清中鼠伤寒沙门氏菌脂多糖(LPS)特异性IgG2a显著升高。相反，在接种后6周和13周可检测到内毒素特异性IgG1。免疫后4周和6周，小鼠脾组织中干扰素-γ基因的表达显著增加。而IL-4mRNA的表达在接种后6周明显增加。结果，口服接种CS2022可以保护小鼠免受随后的强毒小鼠的皮下攻击。此外，通过皮下感染途径，接种后至少6周内，接种能够减少单核细胞增多性李斯特菌在肝脏中的定植。此外，免疫后2周、4周和6周免疫小鼠的腹腔巨噬细胞对单核细胞增生性李斯特菌和鼠伤寒沙门氏菌的细胞内杀伤活性均增强。这些结果表明，CS2022口服免疫可促进与巨噬细胞相关的保护性天然免疫，这种快速保护性免疫也可能足以在小鼠免疫后至少6周内消除沙门氏菌以外的某些细胞内病原体的定植。

The Lon protease-deficient Salmonella enterica serovar Typhimurium (CS2022) is immunogenic when given via the oral infection route in mice. In the present study, CS2022 continuously resided in the spleen of BALB/c mice with less than 10^2 CFU for up to 6 weeks after a single oral inoculation with 10^7 CFU of salmonellae. A significant increase in the serovar Typhimurium lipopolysaccharide (LPS)-specific IgG2a was detected in the serum at weeks 4, 6, and 13 after inoculation. In contrast, the LPS-specific IgG1 was detectable at weeks 6 and 13 after inoculation. The expression of IFN-γ mRNA in the spleens of these mice was significantly increased at weeks 4 and 6 after inoculation. In contrast, the expression of IL-4 mRNA was increased at week 6 after inoculation. As a result, oral inoculation with CS2022 protected mice against subsequent subcutaneous challenge with the virulent serovar Typhimurium. In addition, the inoculation was able to reduce the colonization of Listeria monocytogenes in the livers for at least 6 weeks after inoculation when it was administered via the subcutaneous infection route. Furthermore, peritoneal macrophages isolated from immunized mice at weeks 2, 4, and 6 after inoculation led to an increase in the intracellular killing activity against L.monocytogenes as well as against serovar Typhimurium. These findings suggest that oral immunization with CS2022 promoted the protective innate immunity associated with macrophages, and this rapid protective immunity might also be sufficient to eliminate the colonization of certain intracellular pathogens other than Salmonella for at least 6 weeks after immunization of mice.